The invention disclosed herein was made during the performance of work under a research grant from the United States Public Health Service.
Actinomycin drugs have been known since the early 1940's when Waksman and Woodruff isolated and described the first actinomycin. The actinomycins are a group of antibiotics that occur naturally as metabolic products of the growth of Streptomyces antibioticus (formerly known as Actinomyces antibioticus) and related species, such as S. chrysomallus, S. parvullus.
The antinomycins are orange to red antibiotics, that are highly toxic to most animal species. However they have found usage in nontoxic dosages principally because of their antineoplastic effects. Several actinomycins, specifically, Actinomycin D (or C.sub.1), and C.sub.3 are highly effective chemotherapeutics in the treatment of Wilms' tumor, trophoblastic tumors and rhabdomyosarcoma. Recently, further successes have been achieved by utilizing the actinomycins in combination with other drugs against various types of neoplastic disease.
In addition, it has been determined that the actinomycin molecule binds to desoxyribonucleic acid (DNA) and thereby interferes with the synthesis of ribonucleic acid (RNA). This phenomenon has also given rise of the use of actinomycins as a tool in the study of molecular and cell biology.
Structural studies have revealed that the actinomycin molecule is a chromopeptide, i.e., the molecule contains a chromophore moiety that is linked to a peptide moiety.
The chromophore has been shown to be 3-amino-1, 8-dimethyl-2-phenoxazone-4, 5-dicarboxylic acid derivative, or: ##STR1## more commonly referred to as "actinocin". The chromophore at its two carboxyl sites in turn carries, as amides, two pentapeptides, whose amino acid sequences may be identical, one with the other, in which case the actinomycin is referred to as an "isoactinomycin"; or, when one pentapeptide differs from the other, as an "antisoactinomycin".
While the peptides may have some differences in their amino acid sequences, the actinomycins invariably have but five amino acid moieties in the peptide chains. In addition the amino acids linked to the 4- and 5-carboxyls by amide bonds are always L-threonine, whose hydroxyl is always lactonized with the carboxyl of the fifth amino acid on the peptide chain. The second amino acid may be D-valine or D-allo-isoleucine, while the third may be L-proline, L-.gamma.-hydroxyproline, L-.gamma.-ketoproline, pipecolic acid (not a naturally occuring actinomycin), or sarcosine. The fourth amino acid is always sarcosine, while the fifth may be L-N-methylvaline or L-N-methylisoleucine. As used herein, reference to the free form of an amino acid, e.g., valine, may also be taken to refer to the peptide form, e.g., valyl, etc. Thus, a typical abbreviated structural representation of an actinomycin (D or C.sub.1) is: ##STR2##
The primed numbers to the right are conventional notations indicating the position of the amino acid on the peptide chain.
Although the amino acids within the peptide substituents may be varied only within narrow limits as noted above, the existence of numerous actinomycins is possible because of the unusual structure. Varying the substituents on the chromophore, and/or on the amino acids in the peptide chain can lead to literally hundreds of actinomycin variations and analogues. A great many such variations have been produced both by biosynthesis and by chemical synthesis so that they can be studied as possible valuable materials for chemotherapy applications. Unfortunately, to date, such variations have shown activies of a low order, or no biological activity at all, with a few exceptions of analogues produced by biosynthesis. Generally, however, the early identified naturally produced actinomycins exhibit the highest activities for biological usage.